Memory devices for non-volatile storage of information are widely used in many applications. A few examples of non-volatile semiconductor memories include read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM) and flash memory.
Flash memory is a non-volatile computer storage medium that can be electrically erased and reprogrammed. Flash is a type of EEPROM that is erased and programmed in large blocks. Flash memory costs far less than byte-programmable EEPROM. Flash memory is frequently used in computers, telephones, personal digital assistants, GPS receivers, cameras, and a variety of other electronic devices.
A flash memory cell is similar to a standard metal oxide semiconductor field effect transistor (MOSFET), except the transistor has two gates: a top control gate, below which there is a floating gate insulated on all sides by an oxide layer. The floating gate is between the control gate and the channel, and is electrically isolated by the oxide layer. Any electrons placed on the floating gate are trapped there. When the floating gate holds a charge, it reduces the electric field from the control gate, changing the threshold voltage (VT) of the cell. During read-out, a voltage intermediate between the possible threshold voltages is applied to the control gate, and the MOSFET channel either becomes conductive or remains insulating, depending on the actual VT of the cell, based on the charge in the floating gate. The current flow through the MOSFET channel is sensed, reproducing the stored data.
A single-level flash cell in its default state is logically equivalent to a binary “1” value, because current will flow through the channel under application of an appropriate voltage to the control gate, so that the bit-line voltage is pulled down. A flash memory cell is typically programmed, or set to a binary “0” value by applying voltages to the floating gate transistor in the flash memory cell to cause electrons to be trapped in the transistor's floating gate. Hot carrier injection (HCI) is a basis of operation for a number of non-volatile memory technologies such as EPROM cells and flash cells. A flash memory exploits the principle of HCI by deliberately injecting carriers across the gate oxide to charge the floating gate. This charge alters the threshold voltage to represent a binary “0” value. The flash memory cell is typically erased by applying voltages to the floating gate transistor to remove any charges trapped in the floating gate of the floating gate transistor.
HCI induces floating gate oxide trapping, where fixed charge or carriers are trapped in the gate oxide. This is called HCI damage, which is one of the factors that cause the number of write-erase cycles to be limited. Floating gate oxide trapping may cause a threshold voltage increase during erase and a threshold voltage decrease during write, which affects the ability of the memory cell to have distinct “1” and “0” charge states. As such, HCI damage results in cell current degradation and the closing of the non-volatile memory logic margin window over time. The number of write-erase cycles at which “1” and “0” can no longer be distinguished defines the endurance of a non-volatile memory. There is no satisfactory method for enhancing the endurance of a non-volatile memory cell, in view of the detrimental influence of HCI.